1. Field of the Invention
The present invention relates to systems for connecting telephone subscriber lines to computer networks, and more particularly to a system for switching a telephone subscriber line from a telephone central office switching system to a computer network.
2. Related Art
With the advent of computer networking and personal computers, public switched telephone networks (PSTNs) are increasingly used to connect computer systems to other computer systems, and to connect computer systems to computer networks, such as the Internet. This creates performance problems for PSTNs, because telephone line usage patterns for communications between computer systems are fundamentally different from telephone line usage patterns for voice connections. One problem arises because computer data calls often last much longer than voice calls, and telephone switching equipment cannot distinguish a data call from a voice call. Consequently, voice calls may be blocked due to the long holding times of data calls. Furthermore, data calls do not use the voice bandwidth efficiently. Most of the time, a data connection is not actually sending data, it is simply reserving the connection in case it might use it.
PSTNs are designed to maximize trunking efficiency when only a small percentage of possible connections are in actual use at a given instant. Voice telephone calls typically last about three minutes, and during this time a guaranteed data transfer rate must be sustained. In contrast, communications between computer systems, for example email, do not require a continuous transfer rate; interruptions in the flow of data which would render speech signals unintelligible are acceptable for many computer communications.
Furthermore, existing PSTNs were designed assuming a lengthy set up time followed by a large amount of voice data being transferred. These assumptions do not work well for computer connections, where many smaller data transfers are spread out through a longer session. The call set up time in a PSTN is very long relative to the length of the individual data transfers. In order to minimize latency caused by call set up time, most computer users leave their telephone connections off hook for the entire time of the session, which may last several hours. This problem is amplified by the fact that PSTNs typically bill a flat rate for residential services. Hence, there is no time-based cost for lengthy data connections, and no incentive to limit connection time.
As PSTNs are increasingly used to support data calls between computer systems, the long session times for data calls often degrades the performance of PSTNs. There is a problem with "switch holding time." PSTN switches are designed for an average three minute holding time of a voice call. While data calls last much longer, averaging about eighteen minutes. The result is that switches begin to block because of the long-lived data connections, and additional calls cannot be completed. This blocking unfortunately applies to all calls. Hence, ordinary voice telephone calls cannot be completed because they are blocked by data calls.
One solution to this problem is to design a system around a device known as a POTS (Plain Old Telephone Service) splitter, which splits a signal on a telephone subscriber loop into two pieces; a higher-frequency data component which is routed through a data network, and a lower-frequency voice component which is routed through a central office switching system. However, a POTS splitter cannot separate lower-frequency voice band modem signals from ordinary voice signals in order to route them through a data network.
What is needed is an efficient method of separating data calls from voice calls, so that data calls may be switched from a PSTN to a data network.